1. Field of the Invention
The present invention relates to a digital information reading method and apparatus and, more particularly, to a method and apparatus for correctly reading a digital code in a signal received with interference.
2. Description of the Prior Art
In certain television broadcasting techniques, such as "Pay-TV" systems, the video signal is scrambled in a predetermined manner before it is broadcast. For example, the phase of a particular portion of the signal can be inverted at a predetermined point in every horizontal interval. Provided the exact scheme of scrambling is known, the signal can be unscrambled at the receiver and converted into the original video signal. Because the scrambled video signal is useless without knowing the exact scrambling scheme and without having an unscrambling device, it has been possible to avoid unauthorized use of the transmitted signal by persons who are not subscribers to the Pay-TV system.
The required information for decoding the scrambled signal is commonly contained in a digital key code broadcast as part of the video signal. The television receiver reads the key code and unscrambles the video signal accordingly. However, interference can cause level changes in the signal and effectively mask the key code, the identity of which is of course required to unscramble the signal. A particularly pervasive problem preventing proper reading of such key codes is the type of video interference called "ghosts".